Notes on Pressure

Pressure (symbol: p or P) is a measure of force distribution over any surface associated with the force. Pressure is a surface phenomenon and it can be physically visualised or calculated only if the surface over which it acts is specified.

Pressure may be defined as the normal force acting on unit area of the surface. However a more precise definition of pressure, P is as below by the formula:

pressure formula

‘F’ is the normal force acting normal, ‘A’ is the area of the surface in contact, ‘a’ is the limiting area which will give results independent of the area. This explicitly means that pressure is the ratio of the elemental force to the elemental area normal to it.

The force (dF) in the normal direction on the elemental area dA due to the pressure P is

dF = P.dA

The unit of pressure in the SI system is N/m2 also called Pascal (Pa). As the magnitude is small kN/m2 (kPa) and MN/m2 (MPa) are more popularly used as pressure units. The atmospheric pressure is approximately 105 N/m2 and is designated as ‘bar‘. This is the most popular unit of pressure. In the metric system of the popular unit of pressure is kgf/cm2. This is approximately equal to the atmospheric pressure of 1 bar.

Instruments that can measure the pressure of various fluids (gases and liquids) are listed below:

  1. Pressure gauge
  2. Vacuum gauge
  3. Manometer
  4. Barometer

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